1. Field of the Invention
The present invention relates generally to sulfur-tolerant aromatization catalysts and methods of making and using same, and more particularly to a catalyst comprising a Group VIII metal associated with an L-zeolite wherein the L-zeolite has been incorporated with at least one rare earth ion.
2. Brief Description of Related Art
Pt/L-zeolite catalysts are effective catalysts for the aromatization of aliphatic and cycloaliphatic hydrocarbons such as the aromatization of n-hexane to benzene. Pt/L-zeolite catalysts' ability to achieve high aromatization yields was discovered by Bernard et al. in 1980. As opposed to other standard reforming catalysts, the alkane aromatization on Pt/L catalysts is monofunctional, that is, because the support is non-acidic, the reaction occurs on the metal, i.e., the reaction occurs on the Pt metal sites. Indeed, any residual acidity on the zeolite is detrimental to the catalytic properties of the Pt/L-zeolite catalyst.
The L-zeolite support plays a very important role in determining the properties of Pt/L-zeolite catalysts even though it is not directly involved in the catalytic reactions. When other basic molecular sieves, such as cation-exchanged mordenite, omega, X, or Y zeolites, are used as supports the aromatization selectivity of the catalyst is significantly lower. It is also known that exchanging the cations in the zeolite has a dramatic effect on the catalysts' catalytic properties. For example, if K is replaced by Ba in the catalyst, the selectivity of the Pt/BaL-zeolite was significantly higher, whereas replacing the K with Mg resulted in a Pt/MgL-zeolite catalyst having poor selectivity and stability. In order to justify these results, it was previously shown that replacing K with Mg results in the zeolite having an increase in acidity. This research resulted in the discovery of unique materials which further led to the development of a commercial process for the production of benzene and other aromatic hydrocarbons. In recent years, increasing environmental restrictions relating to the shipping and handling of aromatic hydrocarbons has led to an increased desire for the economical and safe production of aromatic hydrocarbons on site.
After their discovery, it was realized that Pt/L-zeolite catalysts are extremely sensitive to "sulfur poisoning". The high sensitivity of the Pt/L-zeolite catalysts to the presence of even minute amounts of sulfur in the hydrocarbon feed has therefore severely limited their industrial applications as almost all hydrocarbon feeds contain some amount of sulfur. For example, it has been shown that a Pt/BaKL catalyst, which in a sulfur-free environment is able to maintain a high yield of aromatic hydrocarbons for 12 months, became inactive within several days when 1 ppm of sulfur was added to the feed. Similarly, It has been demonstrated that sulfur is a potent poison for these catalysts in that both feed conversion and selectivity to benzene is greatly decreased if 0.2 ppm of sulfur is added to the hydrocarbon feed. It was further shown that a sulfur-free hydrocarbon feed running at 510.degree. C., exhibited an 8% decrease in percent conversion from 90% to 83% conversion during the first 100 hours of run. By contrast, a run conducted in the presence of only 0.2 ppm of sulfur, which was added to the feed as thiophene, resulted in a 67% decrease in percent conversion from 90% to 30% conversion during the first 100 hours of run. Similar results were obtained when selectivity was compared. For the sulfur-free run, the selectivity dropped from 75% to 68% while the sulfur enriched run resulted in selectivities around 30%. Therefore, a sulfur tolerant catalyst became increasingly necessary.
The high sulfur sensitivity of Pt/L-zeolite catalysts is still a subject of controversy. By analyzing the amount of sulfur deposited and the corresponding drop in activity, researchers concluded that the deactivation by sulfur was due to a progressive loss of Pt active sites by blocking of the zeolite channels rather than to a modification of the catalytic (electronic) nature of the catalyst. It was observed that metal agglomeration occurred in an amount that roughly corresponded to the size of the pore mouth.
In a similar, but independent work conducted on Pt/BaKL catalysts, Vaarkamp et al. also investigated the effects of minute amounts of sulfur in the n-hexane feed. They observed that when the aromatization rate decreased to 30% of its original value, the size of the Pt clusters, as measured by EXAFS analysis, increased from 5-6 atoms in the fresh sample to 13 atoms. It was therefore proposed that sulfur was located at the metal-zeolite interface, thus deanchoring the particle and promoting sintering. It is important, therefore, to understand that the deactivation of the Pt/L-zeolite catalyst by sulfur requires the removal of the sulfur, the redispersion of the Pt particles, and the relocation of the Pt particles inside the channels of the zeolite. Finally, a different explanation has been proposed: it has been postulated that the high aromatization activity of Pt/KL catalysts is due to a direct participation of the K.sup.+ ions with sulfur, thus inhibiting the K.sup.+ ions' ability to promote aromatization.
There have been several attempts to increase the sulfur tolerance of the Pt/KL catalysts. For example, researchers at Sun Co. patented a composition of matter that includes the addition of Ni to Pt/KL catalysts. The resulting PtNi/KL catalyst reduces the rate of Pt particle growth in the presence of sulfur and exhibits a moderate improvement in the rate of deactivation by sulfur. Recently, Fang and co-workers proposed that Pt/KL catalysts modified by Tm.sub.2 O.sub.3 exhibit aromatization activity and sulfur resistance which is higher than that of unmodified Pt/KL catalysts. The preparation method reported by these authors, however, results in a catalyst that exhibits a poorer performance than that of the catalysts of the present invention.
Thus, it is an object of the present invention to provide a sulfur-tolerant Pt/L-zeolite catalyst for the conversion of aliphatic and cycloaliphatic hydrocarbons and combinations thereof.
It is yet a further object of the present invention to provide a method for producing a sulfur-tolerant Pt/L-zeolite catalyst for the conversion of aliphatic and cycloaliphatic hydrocarbons and combinations thereof.
It is still a further object of the present invention to provide methods for the reformation, dehydrogenation, and aromatization of aliphatic and cycloaliphatic hydrocarbons and combinations thereof via a sulfur-tolerant Pt/L-zeolite catalyst.
These and other objects of the present invention will become apparent in light of the attached specification, claims, and drawings.